<div id="map"></div>
<script>
    var map = new mapboxgl.Map({
        container: 'map',
        style: 'mapbox://styles/mapbox/streets-v11',
        center: [-88.13734351262877, 35.137451890638886],
        zoom: 4
    });

    map.on('load', function() {
        var layers = map.getStyle().layers;
        // Find the index of the first symbol layer in the map style
        var firstSymbolId;
        for (var i = 0; i < layers.length; i++) {
            if (layers[i].type === 'symbol') {
                firstSymbolId = layers[i].id;
                break;
            }
        }
        map.addSource('urban-areas', {
            'type': 'geojson',
            'data':
                'https://d2ad6b4ur7yvpq.cloudfront.net/naturalearth-3.3.0/ne_50m_urban_areas.geojson'
        });
        map.addLayer(
            {
                'id': 'urban-areas-fill',
                'type': 'fill',
                'source': 'urban-areas',
                'layout': {},
                'paint': {
                    'fill-color': '#f08',
                    'fill-opacity': 0.4
                }
                // This is the important part of this example: the addLayer
                // method takes 2 arguments: the layer as an object, and a string
                // representing another layer's name. if the other layer
                // exists in the stylesheet already, the new layer will be positioned
                // right before that layer in the stack, making it possible to put
                // 'overlays' anywhere in the layer stack.
                // Insert the layer beneath the first symbol layer.
            },
            firstSymbolId
        );
    });
</script>
